In storage tanks for natural gas and hydrogen gas that are mounted in movable bodies such as automobiles, pressure vessels having their tank liners reinforced with a reinforced fiber composite material are utilized in view of the lightweightness of the storage tanks. Regarding the reinforcing fibers used therein, glass fiber, carbon fiber and the like can be used. Among them, carbon fiber has high specific strength and is highly advantageous in weight reduction of pressure vessels. Thus, carbon fiber is suitably used in storage tanks for hydrogen gas where higher pressure resistance performance is required as compared with storage tanks for natural gas.
A pressure vessel reinforced with a reinforcing fiber composite material is generally produced by filament winding molding (FW molding). That is, FW molding is a molding method of paralleling one or plural reinforcing fiber bundles, and during that process, continuously winding the reinforcing fiber bundles around a rotating tank liner at a desired tension and at a desired angle, while supplying a matrix resin to impregnate the reinforcing fiber bundles with the matrix resin. For the reinforcing fiber composite material, a tow prepreg obtained by impregnating a reinforcing fiber bundle with a resin in advance may also be used instead of the reinforcing fiber bundle. In this case, during the process of paralleling one or plural reinforcing fiber bundles, supply and impregnation of a matrix resin is not carried out, and the reinforcing fiber bundles are wound around a rotating tank liner at a desired tension and at a desired angle.
Regarding the matrix resin for the reinforcing fiber composite material that reinforces a pressure vessel, epoxy resin compositions having superior properties and good handleability are generally used. In regard to FW molding, the matrix resin for a reinforcing fiber composite material that reinforces a pressure vessel needs to be supplied to impregnate reinforcing fiber bundles during the process. Furthermore, also in the case of using a tow prepreg, it is necessary for the tow prepreg to have satisfactory reelability, processability, and drape properties. Accordingly, the matrix resin in the reinforcing fiber composite material that reinforces a pressure vessel needs to have very low viscosity compared with general epoxy resin compositions. From the reasons described above, acid anhydrides are widely used as curing agents for the epoxy resin composition (Patent Document 1, Patent Document 2, and Patent Document 3). An acid anhydride is a low-viscosity liquid curing agent, and can lower the viscosity of an epoxy resin composition.
However, an epoxy resin composition that has used an acid anhydride has a short pot life, and definitely cannot be used in intermediate materials such as a tow prepreg. Furthermore, even in a case in which an epoxy resin composition using an acid anhydride has been supplied during the process of FW molding, there is a need to diligently carry out maintenance of many facilities such as a kiss roll or a die for supplying a resin, a resin bus or a resin tank for collecting and holding a resin, and a pump or a piping for transporting a resin. Thus, this has been a cause for significantly deteriorating productivity.
Furthermore, when a solid curing agent such as dicyandiamide is used, an epoxy resin composition having a longer pot life compared with the case of an acid anhydride is obtained; however, there is a problem that the viscosity of the epoxy resin composition is increased. Also, when a solid curing agent is further used, voids are prone to be generated in the reinforcing fiber composite material, and the voids may cause deterioration of the performance of a pressure vessel, or the like (Patent Document 4).